On Tue, 27 Oct 2020 at 21:12, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Tue, Oct 27, 2020 at 12:25 PM Geert Uytterhoeven
> <geert@xxxxxxxxxxxxxx> wrote:
> >
> > Hi Nick,
> >
> > CC Josh
> >
> > On Mon, Oct 26, 2020 at 6:49 PM Nick Desaulniers
> > <ndesaulniers@xxxxxxxxxx> wrote:
> > > On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
> > > <geert@xxxxxxxxxxxxxx> wrote:
> > > > On Mon, Oct 26, 2020 at 6:39 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > > > > On Mon, 26 Oct 2020 at 17:01, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > On Mon, Oct 26, 2020 at 2:29 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > On Mon, Oct 26, 2020 at 1:29 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > > I.e. including the ".eh_frame" warning. I have tried bisecting that
> > > > > > > > warning (i.e. with be2881824ae9eb92 reverted), but that leads me to
> > > > > > > > commit b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section
> > > > > > > > placement"), which is another red herring.
> > > > > > >
> > > > > > > kernel/bpf/core.o is the only file containing an eh_frame section,
> > > > > > > causing the warning.
> > >
> > > When I see .eh_frame, I think -fno-asynchronous-unwind-tables is
> > > missing from someone's KBUILD_CFLAGS.
> > > But I don't see anything curious in kernel/bpf/Makefile, unless
> > > cc-disable-warning is somehow broken.
> >
> > I tracked it down to kernel/bpf/core.c:___bpf_prog_run() being tagged
> > with __no_fgcse aka __attribute__((optimize("-fno-gcse"))).
> >
> > Even if the function is trivially empty ("return 0;"), a ".eh_frame" section
> > is generated. Removing the __no_fgcse tag fixes that.
>
> That's weird. I feel pretty strongly that unless we're working around
> a well understood compiler bug with a comment that links to a
> submitted bug report, turning off rando compiler optimizations is a
> terrible hack for which one must proceed straight to jail; do not pass
> go; do not collect $200. But maybe I'd feel differently for this case
> given the context of the change that added it. (Ard mentions
> retpolines+orc+objtool; can someone share the relevant SHA if you have
> it handy so I don't have to go digging?)
commit 3193c0836f203a91bef96d88c64cccf0be090d9c
Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Date: Wed Jul 17 20:36:45 2019 -0500
bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()
has
Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code")
and mentions objtool and CONFIG_RETPOLINE.
> (I feel the same about there
> being an empty asm(); statement in the definition of asm_volatile_goto
> for compiler-gcc.h). Might be time to "fix the compiler."
>
> (It sounds like Arvind is both in agreement with my sentiment, and has
> the root cause).
>
I agree that the __no_fgcse hack is terrible. Does Clang support the
following pragmas?
#pragma GCC push_options
#pragma GCC optimize ("-fno-gcse")
#pragma GCC pop_options
?
